Is a 144cc big bore kit worth it on a 125?

Are you looking for that little extra power to overcome a certain jump? Or is it because you want to prove to your four-stroke friends that two-stroke dirt bikes still have what it takes? Either way, a heavy gauge kit can help you get there.

The YZ125s are the 125 most common in the US, and for good reason. Yamaha is the only Japanese manufacturer to import two-stroke engines to the United States. They are fast, light, reliable and fun. But, for some of us who just want a bit more of a small-bore two-stroke engine, an improvement in displacement is probably at the top of the mod list.

Just because it’s 144cc doesn’t mean it’s 144 …

You can get a boring 144cc cylinder, but to run like a 144 you will need a proper adjustment. That is why you will hear both positive and negative feedback from people modifying their 125 to 144. Most of the time, it is the people who buy only one cylinder kit that give a bad feedback. This is because the cylinder, carburetor, and exhaust are not properly adjusted for the extra cc’s. When I say a cylinder kit, I mean one that you can buy from a company like Athena and just screw it in.

You can’t go faster without more gas

Too many people think they can make their dirt bike faster by bolting parts together and doing nothing else. Motocross bikes are high-tech racing machines, so it is essential that they are well tuned. If you don’t tune a bike after putting in a “jump” part, it will probably perform worse and possibly break down the road shortly. More power requires more gasoline, so if the carburetor doesn’t feed enough fuel to the cylinder, it won’t work as it should. If your bike was injected correctly before I modified it then it will run lean. You will have to get on the main jet as a minimum. A change in the position of the pilot jet and needle clip may also be required for optimal performance.

A bigger motor needs a bigger tube

Take a look at the size and shape differences of a 125 to 250 two-stroke pipe if you haven’t done it before. Larger engines require larger tubes to take advantage of the greater displacement. Yes, that means if you are using standard tubing on a 144, you are probably not getting the most out of the engine. Some companies make special pipes for large diameters, but they can cost a bit more (still cheaper than the four-stroke exhaust by a lot). If you want to stick with FMF or Pro Circuit, do a little research on which tubing works well for your dirt bike, because not all tubing and bike combinations will give you the same result.

Build the Ultimate Assassin 250F

Now if you bored your 125 two-stroke and built it by a reputable bike manufacturer, it may be out of this world. In addition to drilling and plating the cylinder, most builders will port the intake and exhaust ports to their liking (either low to mid range or medium to high power for most builders), change the port timing , possibly do some box mods if you ship your entire engine and cylinder head mods if you want more compression (will require race gasoline). As a result, this will make it an ultra-fast small-bore two-stroke engine that will top 250F and hold up to 450 with an experienced rider. Oh yeah, did I mention that most stores can also do this for a relatively low price? That’s right, who needs a high-performance four-stroke when you can eat them with a finely tuned 125/144 …

Athena didn’t know what they were doing …

Although I admit that I have never owned a 125 with a 144cc Athena kit, I have done a lot of research to find out what happens. In fact, I didn’t have to look far to find out if it was worth it or not. Review after review showed that Athena’s top-end kit didn’t improve the stock YZ125 much. They basically took the original cylinder, increased the size of the power valve and ports, and called it good … You can see a slight increase in the dynamometer, but to really feel the effect of the extra cubic centimeters you have to carry it like I would do it with a cylinder of values. Port sizes, shapes, and durations will not be the same as 125cc ports if properly tuned. This is where the reputable two-stroke engine manufacturer comes in. There are builders who really know how to open a 144 to its potential, so don’t settle for less.

Here’s a great example from a test of the Athena YZ144 kit by Motocross Action Magazine. They review it in depth, and in the end their YZ barely makes two extra ponies, and that’s with a GYTR tube and muffler made for the 144! If it’s tuned right I honestly think it should be able to produce 5-7 hp more than a stock 125.

-Tom Stark

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